Combustion systems are usually designed to burn fuel that is characterized in that one or more properties of the fuel relevant to combustion of the fuel lies within a range of values. The combustion system generally performs at its highest efficiency when the fuel characteristics fall within the ranges for which the system is designed. Attempts to combust fuel having one or more characteristics outside the range for that characteristic might encounter equipment limitations that prevent the system from reaching design capacity or might lead to negative side effects resulting in operational problems, higher emissions and increased maintenance. Features that may be affected by attempts to combust fuels having one or more characteristics outside of the ranges for which the system is designed include stability of the combustion process, flame heat release pattern, combustion efficiency, NOx emissions, and air and flue gas fan capacity.
One example of a characteristic for which a combustion system is often designed is the mass flow rate of fuel into the combustion chamber of the system. Most solid fuel fired combustion systems (for example, systems that combust coal in a boiler) use air-swept pulverizers to dry and pulverize the fuel, which is then carried into the combustion chamber and ignited. Since the transport medium for the pulverized fuel is air, the pulverizers and the fuel piping are designed to achieve at least a minimum air velocity to avoid settling of fuel particles in the flowing stream of transport air. This design condition then requires operating the pulverizer with at least a minimum air flow rate, even under conditions of low fuel mass flow rates. Maintaining this minimum air flow rate thus dilutes the air/fuel mixture, under conditions of low fuel mass flow rates, to a degree that stable combustion can not be attained and the burner flame gradually extinguishes. This is typically the case at loads below 30% of the full load capacity of the pulverizer. Practitioners have found it necessary to use auxiliary fuel (such as natural gas or oil) to maintain flame and combustion stability when the fuel is being fed at such low rates.
A combustion method that permits combustion to be maintained with a stable flame, even at fuel mass flow rates below those for which the system is designed, would thus be useful.
Other examples of characteristics for which a combustion system is often designed are the content of inert (i.e. not combustible) matter (whether solid, such as ash and minerals, or liquid, typically water), and the specific energy value of the fuel, i.e. the amount of energy obtainable upon combustion of the combustible matter present per unit mass of combustible matter. Fuels that contain more inert matter than the range of inert matter for which the combustion system is designed, and fuels that have a specific energy value below the range of specific energy values for which the system is designed, when fed into the combustion system, cause many of the problems such as inability to maintain combustion with a stable flame.
Fuels that may lead to such operational problems may nonetheless have an economical advantage over fuels that conform to the design specifications of the system. Thus, a combustion method that permits combustion to be maintained with a stable flame, even with fuel that contains too high a proportion of inert matter or too low a specific energy value for the system, would thus be useful.